The direct oxidation of ethylene to ethylene oxide by molecular oxygen is well-known and is, in fact, the method used currently for commercial production of ethylene oxide. The typical catalyst for such purpose contains metallic or ionic silver, optionally modified with various promoters and activators. Most such catalysts contain a porous, inert support or carrier such as alpha alumina upon which the silver and promoters are deposited. A review of the direct oxidation of ethylene in the presence of supported silver catalysts is provided by Sachtler et al. in Catalyst Reviews: Science and Engineering, 23 (1&2), 127-149 (1981).
It is also well-known, however, that the catalysts and reaction conditions which are best suited for ethylene oxide production do not give comparable results in the direct oxidation of higher olefins such as propylene. The discovery of processes capable of providing propylene oxide by vapor phase direct oxidation in higher yields than are presently attainable thus would be most desirable.
The use of molybdenum-promoted supported silver catalysts as propylene epoxidation catalysts is described in U.S. Pat. No. 5,625,084. While such catalysts are capable of producing propylene oxide at relatively high selectivities when the feedstream contains carbon dioxide, the rate at which propylene oxide is formed (as measured by the concentration of PO in the reactor effluent) unfortunately is quite sensitive to the carbon dioxide concentration. That is, as increasing amounts of CO.sub.2 are introduced into the feedstream, the proportion of reacted propylene being converted to propylene oxide becomes greater but the proportion of propylene in the feedstream which is reacted drops significantly, the net result being that the quantity of epoxide produced per unit volume of propylene processed decreases. It would thus be highly desirable to develop direct oxidation catalysts and processing conditions which will permit propylene oxide productivity to be maintained at high levels over a broad range of carbon dioxide concentrations.
According to U.S. Pat. No. 4,007,135, the addition of a number of elements, in particular one of the group consisting of sodium, cesium, rubidium and potassium in combination with at least one member selected from the group consisting of copper, gold, zinc, cadmium, mercury, niobium, tantalum, molybdenum, tungsten, vanadium, chromium, calcium, magnesium, strontium and barium, improves the properties of alpha-alumina-supported silver catalysts used for the preparation of alkylene oxides and having a silver content between 3 and 15 wt. %. This patent teaches that relatively low concentrations of tungsten (from 0.052 to 2570 ppm by weight, based on the final catalyst) are used for said amounts of silver. However, the patent does not teach which tungsten compounds could be used for such purpose, does not specify the nature of tungsten's alleged effect on the properties of the silver catalysts, and does not provide any working examples showing the use of tungsten-containing catalysts.
U.S. Pat. No. 5,407,888 notes that tungsten-doped silver catalysts have not been used in industrial processes for the preparation of ethylene oxide. According to the patent, the reason for this is that doping with tungsten reduces catalyst activity to such an extent that the high temperatures required for long-term usage of these catalysts cannot be attained in the pressure-water-cooled tube bundle reactors normally used for industrial ethylene oxide production. The patent proposes alternative silver catalysts containing one or more alkali metals and also tungsten as promoters on a porous support substantially consisting of low surface area alpha-aluminum oxide and containing tungsten in a concentration not to exceed 990 ppm weight based on the total catalyst.
Additional publications disclosing tungsten-modified supported silver catalysts include U.S. Pat. Nos. 4,808,738, 4,820,675, 5,011,807, 5,145,824, 5,364,826, 5,380,885, 5,447,897, and 5,502,020, Japanese Kokai No. 81-105,650 (Chem. Abstracts 95:2037182), European Pat. Pub. Nos. 480,538 and 480,539, and International Publication Nos. WO95/01837 and WO96/13493.